The invention relates in general to data communication, and more particularly to Digital Subscriber Line on telephone cable.
Digital Subscriber Line (DSL), a new modem technology, converts existing twisted pair telephone lines into access paths for multimedia and high speed data communications. The most successful Asymmetric Digital Subscriber Line (ADSL) transmits more than 6 Mbps to the subscriber, and as much as 640 kbps in both directions. Such rates expand existing access capacity by a factor of 50 or more without new cabling. ADSL can practically transform the existing public information network from one limited to voice, text and low resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to every home.
An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone line, creating three information channelsxe2x80x94a high speed downstream channel, a medium speed duplex channel and a POTS (Plain Old Telephone Service) channel. The POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS, even if ADSL fails. The high-speed downstream channel ranges from 1.5 to 6.1 Mbps, while the duplex rate ranges from 16 to 640 kbps. Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, the presence of bridged taps and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, ADSL will perform as follows:
One problem is that a real telephone cable has many bridged taps that significantly decrease line length. Another problem is that the flat telephone cable that is used for wiring buildings. Practically these problems render it impossible to provide 6.1 Mbps service to more than 30% of the subscribers. The ADSL home modem is a very complex and expensive apparatus. ADSL Central Office Equipment is very expensive too, because every subscriber line must have its own ADSL office modem. The cost of Central Office ADSL Equipment may be as high as a $300 per subscriber.
The ADSL modem was developed to maximize performance for all available lines. The result is a high performance modem which is not always the most cost-effective solution.
ADSL is not cost-effective due to the following reasons:
DLC""s or remote exchanges connect customers located at distances up to 9000 ft from the exchange. Obviously, full high performance ADSL is not need for such short lines. A more economical solution is required.
Current ADSL requires one dedicated central office (C.O.) unit for every customer. This is a very inefficient concept since most of the time only part of the lines are in use so there always are many central office units that are not in use. This inefficiency increases the cost per line and the space needed for ADSL equipment installation. The same service can be provided with a smaller number of units at the C.O. side together with concentration.
ADSL was designed to maximize the data rates on the connection between the customer and the C.O. However, this is usually not the limiting factor. Current Internet communication is limited today by the Internet""s back-bone so increased ADSL data rates will not be truly translated to higher end-to-end communication. This is why lately a G.lite standard defined lower targets for the data rates.
ADSL today requires very complex and expensive apparatus at both the C.O. and the customer side.
ADSL was defined to maximize data rates without taking into account the characteristics of the various applications. Some applications require a high downstream bit rate with a very small upstream bit rate e.g. high quality video. Other applications require mid range symmetric data rates like video conferencing. Current ADSL supports maximal uplink and downlink data rates both at the same time. The result is a more complex solution that supports more than what is actually needed.
From another point of view, ADSL is a very redundant system, because ADSL was developed for maximal use of bandwidth for telephone cable without considering information sources characteristics.
Another problem of existing ADSL systems is that only one subscriber computer may be connected to an Internet service at the same time.
About 15 million subscribers in the USA have at least 2 computers at home and since it is impossible to connect every home computer to the Internet by the same twisted pair, the existing ADSL service limitation is a clear drawback.
In accordance with the present invention, the disadvantages and problems associated with the ADSL system have been substantially reduced or eliminated. In particular, Source Adaptive Digital Subscriber Line (SADSL) provides data service to a subscriber in three operation modes: asymmetric mode, half-asymmetric mode and symmetric mode, according to the information source characteristics.
The SADSL uses QAM or DMT modulation for transmitting downstream and upstream data. The symbol rate of the upstream signal is several times less then the symbol rate of the downstream signal. SADSL transmits downstream and upstream data on the same twisted pair of telephone cable using time division modulation (TDM). The transmission frame of SADSL includes a downstream time interval for downstream symbols and an upstream time interval for upstream symbols. The transmission frame of SADSL has a constant duration in any operation mode but downstream and upstream time intervals have a different duration in different operation modes. Symbols of the synchronization word and symbols of the frame control word are added to downstream data symbols. Symbols of the acknowledges word are added to upstream data symbols.
In accordance with the present invention, information sources that may be connected to the subscriber end of SADSL are divided into three groups:
1. Asymmetrical information sources. These sources require a high bit rate downstream and produce only output control signals with a very low bit rate.
This group relates to the following situation: the subscriber only receives data from the network server.
2. Half-asymmetrical information sources. These sources receive high bit rate downstream data and produce upstream date with a low bit rate.
xe2x80x83This group relates to the following situation: the subscriber receives data from a network server and transmits data to a network server.
3. Symmetrical information sources. These sources require the same middle bit rate upstream data and downstream data.
xe2x80x83This group relates to the following situation: information can be exchanged between subscribers.
Since the amount of information produced by one subscriber is significantly less than the amount of information produced by a network server, every current and future data service will be related to one of the three groups mentioned above.
The Source Adaptive Digital Subscriber Line may work in one of three operation modes, corresponding to three groups of information sources. Selection of the operation mode is implemented during an initialization phase of the communication session. A communication session means here the cycle of information exchange with the current information source. There are many communication sessions with different sources during one connection to a subscriber.
The SADSL Central Office may support communication of many different home modems located at the same house and in different houses as well. Various upstream and downstream data bit rates can be supported. The initialization process of connection to a subscriber implements selection of data bit rates.
In accordance with the present invention the SADSL communication system includes a plurality of different home modems, located at a plurality of subscriber premises and coupled to a number of different information sources, located at each subscriber premises. Every home modem has an information source analyzer and frame control circuit that changes the transmission frame in accordance with the type of information source connected to the home modem during the current communication session.
The SADSL communication system includes a central office coupled to a plurality of twisted pair subscriber lines. The central office includes a number of central office modems that may operate with any home modem. It should be noted that each C.O. modem could support more than one customer. This functionality is achieved by connecting the C.O. modems to the subscriber lines through an analog concentrator. Every office modem has a home modem type analyzer, a frame control circuit and bit rate adapter. The office modem changes its transmission frame structure and the up/downstream bit rate in accordance with the home modem type and the information source type of the subscriber currently connected to this office modem. The number of office modems is smaller than the number of subscriber lines.
In accordance with one embodiment of the present invention, a communication system comprises a local server. The local server is located at the subscriber end of a plurality of twisted pair subscriber lines, routed to a building or telephone box. The local server includes a plurality of downstream repeaters, coupled to twisted pair subscriber lines. The downstream repeater amplifies the downstream signal and splits the twisted pair subscriber lines into a twisted pair data line and a twisted or flat pair phone line.
In accordance with another embodiment of the present invention, a communication system comprises a central office including a plurality of upstream repeaters coupled to twisted pair subscriber lines. The upstream repeater amplifies the upstream signal and splits each twisted pair subscriber line into a downstream data line, an upstream data line and a twisted or flat pair phone line.
In accordance with another embodiment of the present invention, a communication system comprises multiple line repeaters, coupled to twisted pair subscriber lines. The line repeater amplifies the upstream signal and the downstream signal level. The line repeater includes a frame control circuit that is coupled to a synchronization block. The synchronization block receives the downstream synchronization signal.
In accordance with another embodiment of the present invention, a communication system is comprised of several home modems on the subscriber end, connected to the same twisted pair. Every modem is connected to its own information source. Every home modem has an information source analyzer and a frame control circuit that changes the transmission frame in accordance with the type of information source connected to the home modem, during the current communication session.
Every home modem includes super-frame control circuits that produce different transmission frames for different active home modems.
Control of the super-frame circuits of several home modems is achieved by a Central Office Modem connected to a subscriber line in the current communication session. This means that the Office modem transmits data to one active home modem during, for example, the first transmission frame and to another active home modem, during the second transmission frame. The super-frame includes, for example, 4 transmission frames. Every transmission frame may work in one of three operating modes, according to the connected information source. During the 3rd and 4th frame transmission of the super-frame, data transmission between active home modems can take place. This means that the SADSL system in this embodiment provides not only DSL service but includes home digital network functionality i.e. the ability to communicate between units located at the customer premises side.
In accordance with another embodiment of the present invention, a communication system comprises, at the subscriber ends one of several home modems and a Digital Telephone Set connected to the same twisted pair. The Digital Telephone Set includes a PCM or ADM codec and a frame control circuit that is coupled to a synchronization block. This block receives a downstream synchronization signal. The voice signal is coded by the PCM or ADM codec and is transmitted during a part of the time interval reserved for control data and synchronization signals.
Important technical advantages of the present invention includes the ability of SADSL to adapt characteristics of home and office modems to characteristics of the current information source. This advantage makes it possible to develop simple home modems that satisfy all the needs of a subscriber.
Another important technical advantage of the present invention includes the ability of SADSL to work with different types of home modems and to supports many different downstream and/upstream data bit rates.
The SADSL communication system may service simultaneously a number of cheap home modems with a low data bit rate and a number of expensive home modems with a high data bit rate.
Another important technical advantage of the present invention includes the ability to amplify and split SADSL line signals using a simple repeater. In the SADSL communication system there is no limitation of cable length and bridged taps. Practically every subscriber of the existing telephone network may be connected to the SADSL communication system.
Another important technical advantage of the present invention includes the ability to give service to a plurality of subscriber home modems with a reduced number of office modems. This advantage makes the SADSL system a more economical solution than the existing ADSL system.
Another important technical advantage of the present invention includes the ability to provide DSL service and Home Digital Networking service by the same home modem simultaneously.
Another important technical advantage of the present invention includes the possibility to provide Digital Telephone Service on the same twisted pair that is used for DSL and POTS service. It should be noted that the present invention is line code independent. The invention relates to modems utilizing base band modulation such as PAM, single carrier modulation such as QAM or CAP or multi carrier modulation such as discrete multi tone (DMT).
One important embodiment of the invention is based on some variant of the current ADSL standard. This variant is the special standard defined for the Japanese market and is known as G.dmt annex C. Some slight variations to this standard can set the basis for the implementation of the present invention.